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Hi all, new to the forum.
I currently have a Honneywell c901 thermostat set up to control my combi boiler.

I've been looking to buy a smart thermostat to replace the honneywell but when I removed the panel, I see it only has two wires, I hear there are usually 4 wires or more.

Any advice, is it compatible with just two wires?

I've attached some pics.

Kind regards



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I recommend the nest thermostat. And always recommend that an electrician carries out the installation.

If you go with Nest the thermostat itself can usually use the existing 2 core cable to get the 12V power from the nest heatlink.
The heatlink would be fitted adjacent to the boiler and would connect to the boilers wiring.
The existing thermostat cable will need to be disconnected from the boiler and connect to the 12V terminals of the heatlink.
  • Thread Starter Thread Starter
  • #3
Thanks for the response, most grateful.

I was wondering if there is some sort of plug and play option because there is a connector I can see from the bottom right of the boiler, see pic attached.

The boiler is a alpha eco 2




Obviously a plumbers wiring.........is that a bit of 2.5 twin and earth in there with the earth cut out lol.


Nest is a great bit of kit.
Get a spark to wire it up, it can be expensive if you don’t get it right.
I have had to change boiler control boards where diy guy has wired it wrong and blown the control board.
Not to mention the safety aspect.
  • Thread Starter Thread Starter
  • #6
I have no idea who did the install whether it was a plumber or a spark as it was installed just before I bought the house. Anyone got a rough idea of price to supply and fit a nest or similar system?


it would be better to get a spark in a get some qoutes on fitting a nest .
How long is a piece of string .thats a good qestion .
As said Nest E very easy to wire up, it is battery powered so just two wires, Nest 3rd Generation is not really needed with combi boiler, not sure if second generation is still around.

However first point is how will it connect to boiler? There are two very different methods to connect a thermostat, off/on or modulating, the latter is dependent on your boiler, Nest E and Gen 3 both support OpenTherm, but Bosch boilers don't they have their own special system.

So back to basics, why smart and do you know how modern boilers work?

The idea although often not born out in practice, is the boiler does not turn off/on but changes output to suit the demand, so the thermostatic radiator valve slowly opens or closes so modulated the radiator output, this in turn altered the return water temperature, which in turn also alters boiler output, with Opentherm the TRV head sends wifi signal to hub/thermostat, which in turn alters (modulates) boiler output.

However there are some faults with the system, often the TRV does not connect with thermostat, and once warmer weather arrives the boiler can't turn down any more, so it cycles anyway. Very few oil boilers modulate, and gas varies in some cases 6 kW to 28 kW but others 10 kW to 24 kW so how well the modulating system works is very much down to boiler.

Also this connecting the TRV head to hub, EvoHome it seems has a panel where you can select each room and set each room independently, however reports seem to say you need extra bits to use opentherm? Nest works with Energenie? I have Nest and four Energenie TRV heads and they are interconnected it says, but can't see how, only thing I can see is the phone app for energenie shows the Nest wall thermostat as well, however I am using oil so not opentherm may be different with gas?

Last house was gas, tried geofencing, what a waste of time, it should sense your phone within a set distance and turn on heating, but the anti-hysteresis software means it takes 2 hours to reach set temperature, and I live three minutes from work. The Nest has special bits that should learn want you do, but my Nest thermostat is in the hall, I don't live in the hall, so it does not matter what Nest does, it's the TRV head which gets room to temperature set.

So is it really worth £40 each for the TRV heads? For £15 I can get programmable bluetooth TRV heads, will these do the same job, ask me this time next year when I have done a winter with Nest.

So with simple programmable heads no bluetooth £10 each, would these do same or better job than a wall thermostat which only controls one room?

I have Nest Generation 3 mainly as it is one of the few that will control my domestic hot water in a cistern and central heating, plus it works with energenie mihome TRV heads which I already have, and it only needed two wires between the thermostat and heat link, it suited what I needed, however it has turned on when it should not, hope that now cured, but before I sing its praises, want to try a winter, only been fitted 3 weeks.

But for you Nest E is easy to fit, same two wires, OK you can use it with opentherm, but will still work simple off/on so for DIY fitting very easy.
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